The efficient transmission of data between the integrated circuit (or microchip) and the external world has been the focus of intense engineering for IC package manufacturers over the past several years as signal data rates as well as the number of signals are pushed to the physical limits of electrical technology.
Typical industry standard IC packages such as the ball grid array (BGA) package have been able to keep pace with the data rate and pin-out demands from leading edge microchip designers but are steadily facing an ever more challenging set of criteria for density and data rates in the face of growing power consumption concerns. This is exacerbated by the trend towards multi-processor microchip architectures that must draw ever more data from the external world.
The trend towards optical interconnects in the communications and network industries has been based on the trade-off between distance and data rate. As data rates have increased, optical fiber has replaced copper wire (given the same physical distance) so that the higher speed signals are not degraded. It is this trend that has inspired the concept of “fiber to the chip”, where the ultra-high speed electrical signals between the microchip and the outside world are replaced with optical signals. Both the speed and the density issues can then be addressed into the next decade by allowing the microchip to remain as an all-electrical processing unit and have the optical fiber be the ultimate conduit of high speed data to and from the microchip.
There are many examples where light emitting devices have been coupled to and from optical fibers within electrical packages. Work done by the Photonic and Wireless Device Research Laboratories of the NEC Corporation and described by patents such as U.S. Pat. No. 6,901,185 show unique methods of directing and controlling light signals for compact optical modules. Alternative methods, such as those described by the Intel Corporation in patent applications such as US#2002/0196997 show highly integrated methods of incorporating lasers into microchips within the same packages. Other more aggressive means of directing light into the microchip itself have been demonstrated by Luxtera Inc. and part of their technique is illustrated in patent application US#2004/0156590. This technique uses a modulation effect within the silicon itself to produce optical pulses of light directly from the processing chip.
However, none of these technologies have properly addressed the issues of modularity and industry standard form-factor for the semiconductor market. Most of these competing technologies rely on highly vertically integrated assembly techniques where the optical interface is dependent on several layers of alignment steps including micro flip-chipping and precision pick-and-place alignment resulting in a final package that is very specific to the task of converting between electrical and optical signals. There is no provision for a user defined microchip, such as a microprocessor or a switch, to be placed directly along side an optical-to-electrical or electrical-to-optical converter module within the same package. These technologies also rely heavily on the technical sophistication of the integrated circuit package assembly house to provide optically enabled packages.
The ability to merge optics with the computing power of the microchip in the same package, and have the package conform to all the norms of other standard packages both in performance and assembly methodology will allow advances in computer inter-connectivity.
Additionally, a very significant amount of work in both the standardization and product development of optical fiber connectors has been carried over the past several decades. Numerous methods used for mechanical alignment of optical fibers with other optical fibers or optoelectronic modules for permanent and removable connections have been devised. This effort has culminated in various standard optical connector types and optical housings for standard multimode and single-mode optical fibers as well as plastic optical fibers and specialty optical fibers. It has also produced standard types of multi-fiber optical connectors for density improvements and alignment with 1 and 2 dimensional arrays of light emitting and receiving elements. Examples of standard optical connector housings are the LC, FC, SC, and MPO (among others). These connectors typically use at least one precision-machined or precision-molded part containing the optical fiber(s) such as a zirconia ferrule or micro-molded plastic ferrule. The precision part is typically polished on one end to ensure the tips of the optical fibers are flat (although sometimes rounded or at a tilt angle) and allow a maximum amount of light to be coupled into or out-of the optical fiber. The connector housing that surrounds this precision part usually has an attachment mechanism such as a threaded barrel, a plastic snap or clip, or a spring-loaded “floating” assembly to help direct the optical fiber into the ideal position. The mating housing on an optoelectronic module or a passive optical adapter that the connector is mated with will typically have a complementary set of features, such as a precision-machined hollow barrel or a set of precision-molded dowel pin holes. The housing or adapter will also have a complementary set of mechanical attachment features such as a threaded hole, a plastic notch or groove, or a plastic inner adapter to which the connector housing clasps or screws-on. This clasping mechanism is often spring loaded (in some way—either by actual coiled springs, spring steel or compressible plastic or rubber) and offers a positive mating force between the optical fiber and the optoelectronic module or other optical fiber. This force is used to maintain a constant optical coupling between the two optical fibers as well as offer a certain degree of protection from debris that might infiltrate the interface otherwise.
Most optical connectors include both the precision optical part (zirconia ferrule or micro-molded plastic ferrule) and the mechanical attachment mechanism as a single, complete connector assembly on the end of a fiber optical cable. However, there are some mechanical attachment mechanisms that are offered independently from the precision optical part. Examples of these “external” clips can be found in U.S. Pat. No. 5,721,798—by Kanda et al. entitled “Connection Structure for an Optical Waveguide Device and Method of Fabricating the Same”—as an example of a multi-fiber optical connecting mechanism, and U.S. Pat. No. 4,741,590—by Caron entitled “Fiber Optic Connector”—as an example of a single optical fiber connection mechanism.
Further to this, there are various examples of connector housings that allow optical fiber cables to be mated with optoelectronic modules such that the optical fibers are aligned with lasers or photodetectors. The most notable examples of such housings are the standard optical transceiver products such as the SFP, XFP and XANPAK transceiver form-factors—these parts align to dual-LC terminated optical fiber cables. Examples can be found as product offerings by companies such as Finisar Inc. (http://www.finisar.com), Bookham (http://www.bookham.com/), and Intel (http://www.intel.com/design/network/products/optlical/lc transceivers.htm).
The demand for higher data rates and greater aggregate bandwidths leads to the development of hybrid integrated circuit packages that include optical connector interfaces. This hybrid approach brings the optical signals directly to the silicon microchip inside the package thereby alleviating the considerable design and fabrication challenges of very high speed electrical signaling.
Although there have been many methods described that address the placement and alignment of the light-emitting or receiving optoelectronics within standard and non-standard integrated circuit packages, remarkably few optical connectors and connector housings have been suggested for integrated circuit packages. US Patent Application 2003/0031431—by Kunkel et al. entitled “Assembly for aligning an optical array with optical fibers”—describes a clip design that wraps around the package housing and holds on to the back of the package while pushing the optical connector towards the optical interface of the package. U.S. Pat. No. 6,511,233—by Steijer et al. entitled “Spring Clip”—is a similar concept for clasping an external clip to the package while using a spring clip design to push the optical connector on to the optical interface of the package.